This invention relates to an improved method for heating molded preforms to a temperature at which molecularly oriented containers can be produced by subjecting said heated preforms to a thermoforming orienting step.
It is known to molecularly orient thermoplastics in systems wherein such materials are being blow molded into hollow articles such as containers. Such molecular orientation is highly desirable, when the thermoplastic is of such a nature that orientation can be developed therein, since it can represent an attractive route toward improving the strength properties of the finished container. This feature is particularly important when the formed containers are to be used for packaging pressurized liquids such as carbonated beverages and beer as well as other products which require low permeability and high impact resistance as characteristics of the container.
A system is disclosed in Reilly et al, U.S. Pat. No. 3,754,851 for blowing articles from molded preforms which are brought to orientation temperature in an intermediate conditioning step. In this approach heat is removed from the preform during conditioning and such has become known in the art as a "cool-down" process. It is likewise known to add heat to preforms to bring them up to orientation temperature prior to finish forming as typically disclosed in Gilbert, U.S. Pat. No. 3,787,170 and such technique has become known in the art as a "reheat" process.
Although it has generally been considered less difficult to achieve orientation on a heating cycle (reheat process) than on a cooling cycle (cool-down process), the reheat process is by no means a simple procedure to carry out, particularly in a commercial operation. Some factors which affect the process are the preform thickness, thickness variations within a preform (particularly significant in extrusion blown preforms), heating time and the thermal conductivity of the preform material. Additionally complicating the matter are problems associated with forming containers from elongated tubular preforms having one end closed and made of thermoplastic material, It is extremely difficult to heat such preforms with any degree of uniformity because of the particularly low thermal conductivity of the materials and also because of the difficulty of applying heat to the inside of said preforms. Also, measuring the temperature of the preform both on its inside surface and through its thickness is quite difficult thereby hampering the ability to achieve the conditions necessary to obtain containers having the desired properties.
In order to overcome some of the problems noted above, a variety of processes and apparatus have been designed specifically for the reheat process. For example, Steingiser, U.S. Pat. No. 3.830,893 discloses a method for rapidly heating nitrile preforms to orientation temperature using microwave energy; Seefluth, U.S. Pat. No. 3,445,096 discloses a process and apparatus for heating tubular thermoplastic parisons to orientation temperature by alternately passing the parisons between a heating zone and a constant temperature zone maintained at a temperature just below the melting point of the parisons to distribute heat evenly throughout; and Gilbert, U.S. Pat. No. 3,715,109 who discloses apparatus for rapidly heating preforms to orientation temperature by applying heat to said preforms both externally and internally.
Despite the attention given to the problem of reheating workpieces to the desired orientation temperature as noted above, there still is the need for a simplified technique which will allow for the production of molecularly oriented containers using the reheat system and which is suitable for a continuous commercial operation.